It is known that the optimum characteritics of the air intake and exhaust valves of an engine differ according to the running conditions of the engine. At high speed, for example, a large valve lift and a long valve opening period are required in order to obtain large torque, while at low speed, a comparatively small valve lift and short opening period are required.
Further, if fuel consumption is more important than power such as when the engine is on partial load, for example, an even smaller valve lift and shorter valve opening period are required. To improve fuel performance, the negative intake pressure and pumping loss have to be reduced, and it is therefore necessary to reduce the valve lift and reduce the valve opening period so as to increase the throttle opening for the same torque. Due to these differences, the running conditions of engines such as car engines vary widely, and it was therefore difficult to design the shape of valve drive cams in order to obtain optimum performance for all running conditions.
In Tokkai Sho 63-167016 (Koho) publishied by the Japanese Patent Office, a variable cam engine is proposed wherein several cams with different shapes are provided, and the optimum valve timing is obtained by selecting these cams depending on the engine running conditions.
In such a variable cam engine, in order that the engine output torque does not vary discontinuously, the change-over between cams is made at a certain engine speed chosen such that the output torques of the cams are the same for the same throttle opening.
However, although there does exist an optimum speed for making a change-over between a low speed power cam which gives large torque at low speed and a high speed power cam which gives a large torque at high speed, no such speed exists for making a change-over between an economy cam which emphasizes fuel consumption and has a small output torque over the whole range of speeds, and the power cams. The cam change-over is therefore necessarily accompanied by a torque step.
In general, cam change-overs are made in accordance with operation of the accelerator pedal. If for example the accelerator pedal is depressed when the engine is running on the economy cam, an output torque in excess of the range available from the economy cam is required, and a change-over is then made to either the low speed or high speed power cam depending on the engine speed at that time.
However, as the torques generated before and after the cam change-over are very different for the same throttle opening, a torque shock is produced. To correct this, the driver had to operate the accelerator which seriously affected the drive performance of the vehicle.